Building/Buying a Digital Timer
May 31, 2013 8:34 AM Subscribe
I teach Physics and I would love to have a large, easy-to-see digital timer that would allow me to time various demos (i.e. pendulum swinging, ball falling, etc). I've found basically exactly what I want, but it costs $460!! Can it be bought/built for cheaper? Requirements below the fold.
What I like about the linked timer is that it:
1. Is a physical thing (not a stopwatch program on a computer).
2. It has physical buttons to get it to stop and start.
3. It is big and easy to see.
4. It can measure down to a hundredth of a second.
I'm pretty comfortable with electronics (and have colleagues who are even more comfortable), so I would be happy to build one of these, but I don't have a good idea of where to start with parts.
I'm also open to being told that $460 is a reasonable price because, e.g., a screen like that is super-expensive.
What I like about the linked timer is that it:
1. Is a physical thing (not a stopwatch program on a computer).
2. It has physical buttons to get it to stop and start.
3. It is big and easy to see.
4. It can measure down to a hundredth of a second.
I'm pretty comfortable with electronics (and have colleagues who are even more comfortable), so I would be happy to build one of these, but I don't have a good idea of where to start with parts.
I'm also open to being told that $460 is a reasonable price because, e.g., a screen like that is super-expensive.
Would an iPad with a timer app work? The screen is big, my app shows tenth of seconds, and you could hook it up to a projector.
posted by raisingsand at 8:48 AM on May 31, 2013
posted by raisingsand at 8:48 AM on May 31, 2013
The price for this is not outrageous. While it may be a simple project to prototype on an Arduino, the nice enclosure, power supply, robust cable and button box all add up. Parts alone would be not far from $150.
posted by scruss at 8:58 AM on May 31, 2013
posted by scruss at 8:58 AM on May 31, 2013
Best answer: You could certainly build something using large 7 segment LEDs driven by a microcontroller (ie arduino). In fact it would be an awesome electronic project for your class to be involved in and you could make it as complicated as you wanted (triggering/stopping the clock on events due to sensors, etc). That said if you don't know much about microcontrollers or electronics it would probably take a while to figure out, and you would still be spending +$100 for everything.
posted by Quack at 9:00 AM on May 31, 2013
posted by Quack at 9:00 AM on May 31, 2013
Best answer: If you're interested in building this yourself, it's not too hard. First, you need some 7-segment LEDs: Futurelec has them in 2.3" (same size as the product you linked to) up to 5". Then, get a microcontroller of some kind -- Arduino is probably the easiest to get started with. Here's some Arduino code for a stopwatch program, and here's the 7-segment LED interface library. Feel free to memail me if you need more detailed plans.
On preview, I disagree that parts would add up to $150. I think you could do it under $50 -- less if you use a bare ATMega instead of buying the Arduino board.
posted by bradf at 9:01 AM on May 31, 2013
On preview, I disagree that parts would add up to $150. I think you could do it under $50 -- less if you use a bare ATMega instead of buying the Arduino board.
posted by bradf at 9:01 AM on May 31, 2013
Best answer: So the case size is 6" high, looks like the digits are 2/3 of that. I went to Digi-Key, searched for "7 segment LED", clicked on "Display Modules - LED Character and Numeric", filtered for 4" high digits, and it looks like each character runs $15-22, plus driver circuitry (not too hard, but more current and more pins than you want to drive straight out of a microcontroller).
Given that, if I were doing this and not worrying about my time, I'd head towards something like this Instructables, although 1' LED strips seem to be running about $15 each, which means you're quickly up into $630 for the display portions alone.
I like the Arduino suggestions, but if I were building one of these I'd use an Atmel microcontroller (the same thing the Arduino uses), but put a 32kHz clock crystal on it for really awesome time accuracy. But I also have a programmer and a couple of dev boards I designed myself for this sort of thing lying around.
So, yes, as scruss points out you're going to be in for at least $90 for the 7 segment LED items, a bit more for a driver and the shift registers, $60 for an Arduino, you might want to get a couple of circuit boards rather than just soldering up the shift registers (although it also looks like there might be some I2C addressable drivers at Adafruit that you might be able to solder your larger displays), then you have to physically mount this stuff.
If you're doing it as a learning experience, great, or if you're doing a whole bunch of 'em and can amortize your time to get the thing working over twenty or thirty, great, but if you're doing it to save money and your time is worth something, $460 isn't that far out of line.
posted by straw at 9:10 AM on May 31, 2013
Given that, if I were doing this and not worrying about my time, I'd head towards something like this Instructables, although 1' LED strips seem to be running about $15 each, which means you're quickly up into $630 for the display portions alone.
I like the Arduino suggestions, but if I were building one of these I'd use an Atmel microcontroller (the same thing the Arduino uses), but put a 32kHz clock crystal on it for really awesome time accuracy. But I also have a programmer and a couple of dev boards I designed myself for this sort of thing lying around.
So, yes, as scruss points out you're going to be in for at least $90 for the 7 segment LED items, a bit more for a driver and the shift registers, $60 for an Arduino, you might want to get a couple of circuit boards rather than just soldering up the shift registers (although it also looks like there might be some I2C addressable drivers at Adafruit that you might be able to solder your larger displays), then you have to physically mount this stuff.
If you're doing it as a learning experience, great, or if you're doing a whole bunch of 'em and can amortize your time to get the thing working over twenty or thirty, great, but if you're doing it to save money and your time is worth something, $460 isn't that far out of line.
posted by straw at 9:10 AM on May 31, 2013
Best answer: From the OP's link: "Display Size: 2.3 inches"
2.3" 7-seg LED 6@$1.95 $11.70
74HC595 shift reg 2@$0.65 $1.30
Arduino Duemilanove $20.00
Switches 2@$1.00 $2.00
Total: $35
Plus whatever kind of enclosure you want to put it in. Hammond boxes run from $5 up to $25, and come in plastic, steel, or aluminum.
posted by bradf at 9:15 AM on May 31, 2013 [1 favorite]
2.3" 7-seg LED 6@$1.95 $11.70
74HC595 shift reg 2@$0.65 $1.30
Arduino Duemilanove $20.00
Switches 2@$1.00 $2.00
Total: $35
Plus whatever kind of enclosure you want to put it in. Hammond boxes run from $5 up to $25, and come in plastic, steel, or aluminum.
posted by bradf at 9:15 AM on May 31, 2013 [1 favorite]
Get a 17" LCD monitor for $60 and a BeagleBone Black for $45. Add an HDMI cable for a few bucks from Monoprice.
You'd need to hook up a pushbutton and write a little Qt or Web/Javascript application to do the timer, but you could probably slam out a nice design for $110-$120 and not have to mess with soldering LEDs and stuff.
posted by JoeZydeco at 9:57 AM on May 31, 2013 [1 favorite]
You'd need to hook up a pushbutton and write a little Qt or Web/Javascript application to do the timer, but you could probably slam out a nice design for $110-$120 and not have to mess with soldering LEDs and stuff.
posted by JoeZydeco at 9:57 AM on May 31, 2013 [1 favorite]
Oops: that monitor isn't HDMI compatible. Sorry. You'd have to find a consumer TV that has HDMI input, but you could still get one around that price from eBay or WalMart/Target/Costco.
posted by JoeZydeco at 10:05 AM on May 31, 2013
posted by JoeZydeco at 10:05 AM on May 31, 2013
Response by poster: I knew I could count on you guys. Looks like I can do this for a lot less than $460 and, as a bonus, I get to play with an Arduino.
I recognize that I will probably spend more than $400 of "my time" on this project, but it's actually something I think I'll enjoy doing, so I'll happily spend that time.
posted by Betelgeuse at 10:19 AM on May 31, 2013
I recognize that I will probably spend more than $400 of "my time" on this project, but it's actually something I think I'll enjoy doing, so I'll happily spend that time.
posted by Betelgeuse at 10:19 AM on May 31, 2013
You could use Evil Mad Scientist Laboratories' Alpha Clock Five as a basis. It's based on Arduino so it's easy to hack into doing what you want.
posted by zsazsa at 10:26 AM on May 31, 2013 [1 favorite]
posted by zsazsa at 10:26 AM on May 31, 2013 [1 favorite]
Best answer: Well, I can tell you how we used to do this for high school physics classes in the old days before all your fancy pants digital timers.
For the timer in a falling object experiment we used an old fashioned electric bell powered by a battery. The clapper on the bell strikes at a fixed frequency and interval as a fraction of a second and that becomes your time base.
Then you put a piece of carbon paper between the clapper and the bell. You attach the falling object to a thin roll of paper tape and the paper tape passes between the clapper and the carbon paper. As the object falls, the paper tape dragged behind gets carbon dots each time it is hit by the clapper. The spacing between each dot on the tape is directly proportional to the velocity of the falling object.
You can directly plot the distance between each dot versus time and easily see that distance is proportional to time squared and that velocity is directly proportional to time. So you have derived the basic equations for the mechanics of a falling object, with an unknown constant "g".
Next you need to figure out the value for the constant of acceleration in your equations. For that you need to quantify your time base, the bell clapper. To do that you use a stroboscope. This is constructed of a cardboard disk with 10 small slots cut into it radially like the hands on a clock. The disk is mounted to a stick perpendicular to the center and then spun by your finger through a hole near the stick like a toy propeller. You spin the disk of the stroboscope with your finger until you get a speed that is just enough to cause a freeze-frame appearance of the clapper when viewed through the slits. Then you count the number of revolutions of the disk in a 10 second period using an ordinary clock or watch. From the revolutions per second and the number of slits per revolution, you can calculate the frequency and period of the bell clapper.
Once you know the time period of the clapper, you can go to your paper tape and assign absolute time to the intervals between each dot on the tape. From your measurements of distance and time and the equations of motion you derived, you can calculate the constant of acceleration due to gravity.
Through this process you learn how physics is really done. A lot of advances in physics (and Nobel prizes) involve clever ways of measuring physical properties that cannot be done directly, like the distance to far away galaxies or the size of an atom or the charge of an electron or the speed of light. You also learn about accounting for all the inaccuracies in your measurement methods and how to reduce them.
Now get off my lawn with your damn digital timer.
posted by JackFlash at 12:20 PM on May 31, 2013 [6 favorites]
For the timer in a falling object experiment we used an old fashioned electric bell powered by a battery. The clapper on the bell strikes at a fixed frequency and interval as a fraction of a second and that becomes your time base.
Then you put a piece of carbon paper between the clapper and the bell. You attach the falling object to a thin roll of paper tape and the paper tape passes between the clapper and the carbon paper. As the object falls, the paper tape dragged behind gets carbon dots each time it is hit by the clapper. The spacing between each dot on the tape is directly proportional to the velocity of the falling object.
You can directly plot the distance between each dot versus time and easily see that distance is proportional to time squared and that velocity is directly proportional to time. So you have derived the basic equations for the mechanics of a falling object, with an unknown constant "g".
Next you need to figure out the value for the constant of acceleration in your equations. For that you need to quantify your time base, the bell clapper. To do that you use a stroboscope. This is constructed of a cardboard disk with 10 small slots cut into it radially like the hands on a clock. The disk is mounted to a stick perpendicular to the center and then spun by your finger through a hole near the stick like a toy propeller. You spin the disk of the stroboscope with your finger until you get a speed that is just enough to cause a freeze-frame appearance of the clapper when viewed through the slits. Then you count the number of revolutions of the disk in a 10 second period using an ordinary clock or watch. From the revolutions per second and the number of slits per revolution, you can calculate the frequency and period of the bell clapper.
Once you know the time period of the clapper, you can go to your paper tape and assign absolute time to the intervals between each dot on the tape. From your measurements of distance and time and the equations of motion you derived, you can calculate the constant of acceleration due to gravity.
Through this process you learn how physics is really done. A lot of advances in physics (and Nobel prizes) involve clever ways of measuring physical properties that cannot be done directly, like the distance to far away galaxies or the size of an atom or the charge of an electron or the speed of light. You also learn about accounting for all the inaccuracies in your measurement methods and how to reduce them.
Now get off my lawn with your damn digital timer.
posted by JackFlash at 12:20 PM on May 31, 2013 [6 favorites]
Best answer: FYI - another variation on the paper tape and buzzer is one where you tape thin paper strip to a long pipe mounted plumb and grounded. Then you hang an iron plumb bob from an electro magnet. The plumb bob has a wire coming up to it with house current running to it. When you cut the electro magnet, the bob will fall and sparks will arc through the paper to the pipe. The frequency of spark should be whatever your house current is (ie, 60Hz). You can use the distance between burns on the paper to determine velocity and acceleration.
posted by plinth at 6:46 PM on May 31, 2013
posted by plinth at 6:46 PM on May 31, 2013
Response by poster: And, as a bonus, JackFlash and plinth have given me a great idea for a lab. Thanks, folks!
posted by Betelgeuse at 7:22 PM on May 31, 2013
posted by Betelgeuse at 7:22 PM on May 31, 2013
This thread is closed to new comments.
posted by JJ86 at 8:47 AM on May 31, 2013